Electric arcs can occur when ionized air breaks down into plasma that carries a stream of current between conductors or between a conductor and ground. Electric arcs that are large enough and that occur quickly enough are known as arc flashes (also referred to as flashovers). An arc fault is a type of electrical explosion or discharge that results from such a low-impedance connection through air to ground or another voltage phase in an electrical system. Arc faults generate pressure by expansion due to metal vaporization and rapid heating of air by arc passing through it. An arc creates copper vapor that expands to 67,000 times the volume of solid copper (e.g., 1 in3 of copper vaporizes into 1.44 yd3 of vapor). Air in an arc stream heats up to about 35,000° F. (Surface of the Sun: 8,540° F.). During an arc flash, electrical energy can vaporize a metal conductor carrying the electrical energy, causing the metal to changes from solid state to gas vapor, expanding it with an explosive force.
Arc flashes can cause dangerous explosions. Five to ten arc flash explosions occur in electric equipment every day in the U. S, with injuries so severe that the victims require treatment from a special burn center. Moreover, arc flash explosions likely cause many more less severe injuries that are not part of this five to ten number, because some injured victims are sent only to an ordinary hospital and do not require aid from burn centers. Arc flashes also can cause severe non-burn types of injuries. For example, a pressure wave from an arc flash has the potential to blow out eardrums or damage lungs, and the “flash” itself emits large doses of infrared and ultraviolet light that can blind temporarily or even permanently.
Power systems for electronic weapons systems present one type of major arc flash hazard. Many electronic weapons systems operate from high power DC sources (e.g., 300 to 1000 VDC) and can have energy storage in the range of farads. Arcs lengthen and intensify as voltage and current increase, so the higher voltages used in these system increases the risk and dangerous consequences of arc flashes. Undersea systems, which operate at about 10 kV DC, present similar challenges. New electronic warfare systems require megawatts of power in the hundreds of volts to kilovolts voltage range. Use of these types of high DC voltages increases the risk of arc flash. The new radar requirements recognize this risk and include particular requirements for prevention of shocks and burns, specifying that the power system must have an arc resistant design implemented via construction techniques and features such as adequate conductor spacing, insulated bus work, and high bus ratings (e.g., 250 kVA). The requirements also specify use of elements to mitigate the consequences of an electrical arc by incorporating pressure relief vents and champers to redirect pressure for bus ratings exceeding the bus rating.
Another type of system that can present a significant arc flash hazard includes systems and locations that use high voltage DC, such as industrial computing and telecommunication markets, including large server systems and data centers. These systems increasingly use high voltage DC distribution systems to increase accommodate the high power demands of digital processors, improve efficiency, reduce power distribution losses, and reduce the size and weight of the systems. For example, in the United States, Japan, and selected other countries, at least some computer manufacturers have adopted 360 VDC as their standard input supply voltage. In addition, digital processors continually require more power. Other markets, including military markets, use 750-1000 VDC distribution systems.
The astronomical growth in data center power use—and the huge electrical supply sources they require—creates the potential for bigger and more dangerous electric arcs, and more places where they can cause damage. Damage in such server systems and data centers can have both a huge human toll and a significant financial cost (especially if data is damaged or destroyed). In addition, the drive for higher efficiency and lighter power distribution systems is expected to increase the source voltage of other types of systems. Thus, it is expected that the problems due to high voltage arc flash will become more prevalent, unless techniques are developed to help, detect, prevent, and mitigate for these types of faults.